The invention described in this application may be considered as belonging to the third generation High Voltage Direct Current (HVDC) system. The first generation is centered around the technology of the mercury arc rectifier, the second around the thyristor. Because of the advent of high power and high frequency semiconductor switches with fast turn-off capabilities (e.g. GTO's, MCT's, etc . . . ) the pulse-width modulation (PWM) technique, may be applied for bulk power transmission of the utilities. The exploitation of the PWM technique constitutes the third generation HVDC system.
Because the thyristor generation of HVDC has been developed at considerable costs, industry is not inclined to invest heavily on another new technology if the gains are merely marginal. The invention described in this application not only enables HVDC systems to perform their existing functions better but also it enables tasks to be performed which are not possible with thyristor technology.